1. Field of the Invention
The present invention relates to a node, a correspondent node, a mobility anchor point, and a home agent in a packet communication system comprised of nodes and links, and to a packet communication system and a Path MTU discovery method.
2. Related Background Art
An IP [Internet Protocol] network is comprised of a set of nodes and links directly interconnecting the nodes to each other. The nodes are the generic name of computers, routers for relaying communication between computers, and the like. The links presently available include several types of links such as Ethernet, FDDI [Fiber Distributed Data Interface], ATM [Asynchronous Transfer Mode], and so on. Since the IP network includes these types of links mixed, there exist different types of links on a path from a node of a source (correspondent) to a destination node during execution of data communication.
In packet communication systems for transmitting data in packet units, an MTU [Maximum Transmission Unit] of IP packets is determined for each link and this MTU for each link is called a link MTU. The link MTUs are different among links and, in the case where a link MTU is smaller than a packet size of an IP packet under transmission in the middle of the path, there arises a problem that the packet cannot be transported further. In order to avoid this problem, the IP layer is configured to fragment each IP packet into smaller fragmented packets and transport the fragmented packets onto the link with the small link MTU. In the case of the IP packet being fragmented, however, if even one fragmented packet is lost, it is impossible to recover the original IP packet, which results in loss of the entire IP packet. In order to avoid it, TCP [Transmission Control Protocol] is configured to transmit each packet in small size enough to prevent further fragmentation of the packet, but it results in degradation of transmission efficiency of the network. For overcoming it, IETF [Internet Engineering Task Force] presented a method of transmitting a packet in the minimum link MTU (i.e., Path MTU) among those of links existing on the path from the source node to the destination node and a method of discovering the Path MTU, which are standardized in RFC [Request For Comments].
When the node is a mobile node (hereinafter referred to as MN [Mobile Node]) having the mobility control function for movement in the IP network, there exists a home agent (hereinafter referred to as HA [Home Agent]) for managing global location information of the MN and mobility anchor points (hereinafter referred to as MAPs [Mobility Anchor Points]) for managing local location information of the MN are also set in the IP network. Then a packet to be transmitted to the MN is transported via the HA and MAPs to the MN. When the HA receives a packet addressed to the MN during a period in which the MN stays away from the links under control of the HA and is connected to a foreign link, it forwards the packet to an address registered in the HA and presently temporarily acquired by the MN. On that occasion, the HA adds an IP header with the source being the HA and the destination being the address presently temporarily acquired by the MN, to the packet having been transported thereto. This addition of the IP header to the original packet is called encapsulation. A node carrying out the encapsulation is called an entry point of a tunnel. The method of encapsulating the packet is also standardized in RFC by IETF. Registered in the MAPs is an address locally acquired by the MN in an area where the MAPs are set. When a MAP receives a packet addressed to the MN, it forwards the packet to the address of the MN registered in the MAP. On that occasion, the MAP adds an IP header with the source being the MAP and the destination being the registered address of MN to the packet having been transported thereto. Therefore, the packet addressed to MN is given an IP header every time it passes the HA or a MAP. Accordingly, IP headers are added up to the number of HA and MAPs existing on the path from the source node to the MN. Each node that carries out an encapsulation on the path from the source to the MN in the case of such addition of multiple IP headers as described above will be called an entry point of multiple tunnels.